The present invention relates to a process for preventing the polymerization of readily polymerizable vinyl aromatic compounds during distillation with chemical polymerization inhibitors. More particularly, the present invention relates to a method for inhibiting the polymerization of styrene, substituted styrene, divinyl benzene, and polyvinyl benzenes, during distillation at elevated temperatures under emergency conditions in which the normal polymerization inhibitor system is rendered inoperative.
It is well known that vinyl aromatic compounds such as monomeric styrene, lower alkylated styrene, e.g., alpha-methyl styrene, and the like polymerize readily at elevated temperatures. Accordingly, it is common practice to adopt measures to prevent the polymerization of these compounds during distillation. Typically, these measures comprise pumping a chemical polymerization inhibitor into the distillation columns of a conventional distillation train. To this end, many chemical polymerization inhibitors have been developed which are highly efficacious in prevent the polymerization of vinyl aromatic compounds during distillation under normal situations. There may be mentioned, for example, N-nitrosophenylhydroxyl amine, para-nitroso-N, N-dimethylaniline, N-nitroso diphenyl amine U.S. Pat. No. 3,816,265) a phenothiazine-tertiary butyl catechol (TBC) chemical inhibitor system, N-nitrosomethylaniline, sulfur, TBC, mixtures of sulfur and TBC, dinitro-o-cresol, dinitrophenol, and meta-nitro-parap cresol.
Occasionally, however, an emergency situation develops, such as a power outage, which renders a given distillation facility unexpectedly inoperative. In these instances long periods may ensue during which a vinyl aromatic feed is trapped within the distillation train at elevated temperatures with the absence of any substantial heat transfer from the liquids contained in the distillation column bottoms, and with the inhibitor pumping system rendered inoperative. In the event of a power failure, therefore, a considerable amount of time may lapse during which a vinyl aromatic compound feed is at elevated temperatures with polymerization inhibition provided only by the small quantity of inhibitor present with the vinyl aromatic compound in the distillation train. Such limited quantities of conventional inhibitors, hereinafter referred to as primary inhibitors, are inadequate however to provide sufficient long-term polymerization inhibition to the vinyl aromatic compound feed. The low solubility in the vinyl aromatic compound, the requirement for air activation, and/or the short half-life reduces the efficacy of many of the most effective distillation polymerization inhibitors during prolonged power outage situations. Ocassionally, in fact, the protection provided by the limited quantities of primary inhibitor present within a distillation train subject to such conditions has been so grossly inadequate as to result in complete polymerization within the distillation apparatus. Such a situation represents not only a significant potential danger to the personnel and property at the distillation facility, but the costs in labor and down-time can be very significant where extensive polymerization of the vinyl aromatic compound trapped within the distillation train occurs. Normally, solid polymer can be removed from a polymerization vessel only by a tedious manual operation.
While several polymerization inhibitors are known, such as dinitro-o-cresol and dinitrophenol, which do not suffer from the aforementioned deficiencies, and therefore could be expected to provide reliable prolonged control of polymerization, the acute toxicity which these compounds exhibit militates against their selection as either emergency or primary inhibitors. Of course, the present invention may be utilized with such inhibitors if desired, and such use would still be within the spirit and scope of the invention. Moreover, while many compounds are known to provide prolonged polymerization inhibition under room temperature conditions, such as storage, under distillation at elevated temperatures, these compounds are ineffectual in inhibiting polymerization. It is apparent therefore that the prior art has failed to develop a satisfactory polymerization inhibitor which provides adequate protection at elevated temperatures under emergency conditions. Accordingly, in view of the recent emphasis upon developing safety measures for chemical installations, and particularly in view of several regional power failures which have occurred within the last several years, it would be desirable to provide a method whereby the polymerization of a vinyl aromatic compound within a distillation apparatus at elevated temperatures may be prevented during emergency conditions in which the normal primary inhibitor is ineffective.